Studies on the process of cell division in synchronously dividing sea urchin eggs have been carried out through the preparation of the isolated mitotic apparatus (MA) from the first cleavage. With the aim of relating our investigations on the isolated MA to recent studies on the role of the protein tubulin in the assembly and functioning of the microtubule, we attempted to devise methods for the preparation of this protein from the urchin egg. However, the conventional methods for the preparation of tubulin, when applied to urchin egg extracts, yield instead the protein actin in the form of a co-polymer with a second unknown protein. Investigations are now proceeding along two lines, one concerned with the further purification of actin from this co-polymer, in which considerable success has been achieved, and the second concerned with the identification and role of the unknown protein. These proteins may play an important role in cytoplasmic and/or chromosomal division, and our studies will be extended to investigate these possibilities.